Linear and nonlinear microrheology of dense colloidal suspensions

Wilson, L., Besseling, R., Arlt, J. and Poon, W. C. K. (2006) Linear and nonlinear microrheology of dense colloidal suspensions. In: Dholakia, K. and Spalding, G. C. (eds.) Proceedings of the Society of Photo-Optical Instrumentation Engineers, Optical Trapping and Optical Micromanipulation III. Proceedings of the Society of Photo-Optical Instrumentation Engineers (Spie), 6326. SPIE, San Diego. doi: 10.1117/12.681569

Abstract/Summary

The length and time scales accessible to optical tweezers make them an ideal tool for the examination of colloidal systems. Embedded high-refractive-index tracer particles in an index-matched hard sphere suspension provide 'handles' within the system to investigate the mechanical behaviour. Passive observations of the motion of a single probe particle give information about the linear response behaviour of the system, which can be linked to the macroscopic frequency-dependent viscous and elastic moduli of the suspension. Separate 'dragging' experiments allow observation of a sample's nonlinear response to an applied stress on a particle-by particle basis. Optical force measurements have given new data about the dynamics of phase transitions and particle interactions; an example in this study is the transition from liquid-like to solid-like behaviour, and the emergence of a yield stress and other effects attributable to nearest-neighbour caging effects. The forces needed to break such cages and the frequency of these cage breaking events are investigated in detail for systems close to the glass transition.